1. Field of the Invention
This invention relates to sequence controllers and, more particularly to multi-program sequence controllers with preferential priority program designation capability.
2. Description of the Prior Art
Sequential control circuits, particularly for industry, have, in the past, relied primarily on electro-mechanical relay logic. More recently semiconductor sequences, or hybrid sequencers have appeared on the industrial scene. All of these sequencers suffer from a primary limitation. That is, that once the sequence has begun it cannot be interdicted. This rigidity can and has caused damage to the process or apparatus being controlled. In many practical cases it is desirable to be able to change preferences or priorities during processing. Special problems arise with conventional circuits when a second signal, of the same priority as the first, is introduced for the purpose of pre-empting the sequencing program of the first signal. Similarly, problems arise in the case where the order of priority for two input signals cannot be established until a third signal appears and that signal may occur in the middle of an on-going sequence.
The mere introduction of a PROM to store multiple sequencing programs does not solve the problem of fixed priority during sequencing.
Therefore, it is one object of this invention to overcome all of the shortcomings of the present systems, as described hereinbefore.
It is a further object of this invention to provide a sequence controller with a dynamically changeable program.
It is a still further object of this invention to provide a multiple-sequence controller in which an on-going sequence may be interdicted and replaced by a preferred sequence.